The present invention relates to ceramic bodies formed from partially stabilized zirconia having particular utility as high temperature filters for molten metal, kiln furniture and high temperature catalyst carriers.
Ceramic materials are used in high temperature applications because of their excellent thermal and mechanical properties. Filters for use in molten metal filtration applications are typically produced by a replication process which in its most basic form comprises immersing a porous combustible foam material in a thixotropic ceramic slurry and firing the impregnated foam material to burn off the foam material and produce a porous ceramic body. U.S. Pat. Nos. 3,947,363 to Pryor et al., 4,024,212, to Dore et al., 4,265,659 to Blome, 4,391,918 to Brockmeyer, and 4,610,832 to Brockmeyer illustrate replication processes forming ceramic foam filters. These patents also illustrate the various types of ceramic materials employed in forming the filters.
The Pryor et al. patent for example utilizes a slurry containing alumina, chromia, kaolin, bentonite and colloidal aluminum orthophosphate to form the filter body. The Pryor et al. patent also states that materials such as mullite, zirconia, magnesia and the like may be employed in addition to the alumina and/or chromia components or in substitution therefor to achieve particular properties. The Dore et al. and Blome patents also mention that the thixotropic ceramic slurries used to form ceramic filters may include a wide variety of ceramic materials such as alumina, chromia, zirconia, magnesia, titania, and silica.
U.S. Pat. No. 4,760,038 to Kinney, Jr. et al. relates to a ceramic composition having enhanced thermal shock resistance and resistance to high temperature degradation. The ceramic composition has alumina as a principal ingredient and controlled additives of one or more of zirconia, titania, or maganese oxide.
High temperature applications such as kiln furniture and foundry applications require the use of ceramic material possessing certain combinations of physical properties. For example, in kiln furniture applications, the ceramic material must be substantially chemically inert with such wares as electronic components containing lead based titanates. The ceramic material must also tolerate many high temperatures cycles up to 1250.degree.-1480.degree. C. and even as high as 1870.degree. C. Still further, it must have a low thermal mass for faster firing and energy saving. In foundry applications, the ceramic must tolerate a very severe temperature up-cycle such as 1090 -1540.degree. C in 5 seconds. The ceramic material must also have a relatively high purity and relatively high permeability (10 ppi, vertical cut foam required) as well as be chemically inert with the metal alloys being processed.
Zirconia is a ceramic material which lends itself to such high temperature applications because of its excellent mechanical, thermal and chemical properties. Unfortunately, it is a difficult material to work with because in an unstabilized form, it goes through several phase changes when exposed to high temperatures. At room temperature, unstabilized zirconia is monoclinic. As it is heated to a temperature in the range of 1000.degree. C. to 1200.degree. C., it undergoes a phase change to a tetragonal structure. At higher temperatures, on the order of approximately 2000.degree. C., it undergoes yet another phase transformation to a cubic structure. When it cools down, the zirconia goes back through the phase transformations. Potentially detrimental volume changes occur during these phase transformations. For example, zirconia contracts when it goes from the monoclinic to the tetragonal phase and expands during the reverse phase change. It has been found that zirconia can be a viable ceramic material for high temperature applications only if the amount of zirconia which goes through these phase transformations is strictly controlled.
Accordingly, it is an object of the present invention to provide a ceramic body formed from zirconia which is suitable for use in high temperature applications.
It is a further object of the present invention to provide a ceramic body as above which exhibits excellent thermal shock resistance and high temperature strength properties.
It is still a further object of the present invention to provide a ceramic body formed from partially stabilized zirconia.
It is yet a further object of the present invention to provide a process for forming the above ceramic body.
These and other objects and advantages will become clearer from the following description.